The invention is related to an actuator, comprising a housing with a motor and a screw mechanism, said screw mechanism comprising a nut and a screw one of which is rotatably supported with respect to the housing by means of a screw mechanism support allowing relative rotation of the screw and the nut with respect to the longitudinal screw mechanism axis, and an actuating member connected to the screw mechanism by means of an actuating member connection, such that on relative rotation of the nut and screw the actuating member is displaced linearly with respect to the housing.
Such actuator is known. The motor, which can be electric, hydraulic etc., rotates either the nut or screw which in turn imparts a translational movement to the other thereof. As an alternative, the nut or screw may be fixed completely, the other in that case carrying out both a rotational and a translational movement.
The nut and screw are each provided with screw threads which, in the most simple embodiment, are in direct engagement. In order however to reduce the internal friction of the screw actuator, often the screw and nut engage each other through balls or grooved rollers which are accommodated in the correspondingly shaped screw threads of nut and screw.
The latter embodiment in particular provides a high efficiency, leading to reduced power consumption. Moreover, the load capacity with respect to linear loads is excellent. However, ball or roller screw mechanisms are rather vulnerable to deviating loadings such as transverse loadings.
The object of the invention is to provide an actuator of the type referred to before, which does not have this drawback of vulnerability with respect to transverse loadings. This object is achieved in that the screw mechanism support and the actuating member connection allow rotations having at least one axis transverse with respect to the longitudinal screw mechanism axis (swivelling motions).
As the screw mechanism at both ends is now loaded through connections allowing transverse rotations, any bending moments and transverse loadings are reduced or even completely absent.
The screw mechanism support may comprise a ball swivel with transmission splines which do not allow rotations with respect to the longitudinal axis of the actuator. Such ball swivel may be carried out in analogy with a homokinetic joint. In this sense, the screw mechanism support is stiff with respect to rotations around said longitudinal axis in order to (quasi-) immobilize the stator of the motor against rotating.
In case also the motor is supported with respect to the housing through the screw mechanism support, the transmission between the motor and the screw mechanism is not exposed to relative movements. Thus, in case the motor engages the screw mechanism through a gear reduction, no specially shaped gears are necessary.
In the alternative, the motor may be fixed with respect to the housing. In that case, relative transverse rotations may occur between the motor and the screw mechanism, which necessitate the application of specially rounded gears or the application of a special coupling.
The screw mechanism support and the actuating member connection may comprise ball swivels with or without transmission splines. Such ball swivels do allow transverse rotations in any direction.
Preferably, the nut is connected to or integrated with the outer ring of a screw mechanism support bearing, the inner ring of which being connected to or integrated with the ball swivel. Also, the screw may be connected to or integrated with a ball swivel which is in turn connected to or integrated with the actuating member.
The actuating member is laterally guided with respect to the housing so as to limit the transverse rotations and movements of the screw mechanism. This guidance can be obtained by e.g. laterally extending spring means 27, or by accommodating the actuating member in a bore or slit etc.
The invention is also related to a brake calliper for a disc brake, comprising a calliper housing which supports at least two opposite brake pads between which a brake disc can be accommodated, and an actuator as referred to before, said actuator comprising a housing with a motor and a screw mechanism, said screw mechanism comprising a nut and a screw one of which is rotatably supported with respect to the housing by means of a screw mechanism support allowing relative rotations of the screw and the nut with respect to the longitudinal screw mechanism axis, and an actuating member connected to the screw mechanism by means of an actuating member connection, such that on relative rotation of the nut and screw the actuating member is displaced linearly with respect to the housing.
According to the invention, the screw mechanism support and the actuating member connection allow rotations having at least one axis transverse with respect to the longitudinal screw mechanism axis 28 (swivelling motions).
The actuating member is accommodated in an opening or bore etc. in a transverse wall of the brake calliper housing or fixed directly to the brake pad, said actuating member extending through said opening and carrying a brake pad at its protruding end. Thus, (under misaligned brake forces) the movements of the actuating member, and thereby of the actuator can remain within acceptable limits.